1. Field of the Invention
The present invention relates to an apparatus and method for measuring thermal diffusivity using the flash method, and more particularly, to an apparatus and method for measuring thermal diffusivity using the flash method, in which the thermal diffusivity of a sample can be measured accurately using the method of determining the resistance factor of coated graphite on the sample and measuring the apparent thickness of the coated sample.
2. Background of the Related Art
The accurate measurement of thermo-physical properties such as thermal diffusivity, specific heat and thermal conductivity is of prime importance in heat transfer analysis and application technologies in engineering. Especially, as new highly functional solid material and heat transfer medium are fast exploited with rapidly developing industries, establishment of a reliable and accurate measurement technique of the thermo-physical properties is highly demanding.
In the conventional thermal conductivity measurement technique in a steady state, the contact resistance between the test sample and the measuring tool causes non-negligible error, and the conventional measurement requires relatively long measurement time. In contrast, since the flash method is a non-contact method, the thermal diffusivity is measured in a short time, the sample size can be made small and it is easy to acquire the data. In addition, it enables the diffusivity measurement in a wide temperature range from room temperature to 2,000 degrees Celsius.
Graphite coating of sample is a process that is fundamental in thermal diffusivity measurement by the flash method. It increases both the absorbance of flash energy on the front surface and the intensity of the infrared light which is emitted from the rear surface. Moreover, the graphite coating plays an important role in decreasing the surface roughness. However, the additional graphite coating increases the thermal resistance of the sample. This has become the most critical factor of an error occurring when measuring the thermal diffusivity.
As a solution to this problem, Hasselman, et al. recommends that materials with a high thermal diffusivity, such as aluminum, must have an ideal thickness of 3 mm or more and every measurement sample must have an optimal thickness. However, according to this method, it is practically impossible to set an optimal thickness for every material because there is a need for the development of new materials and special functional materials with excellent thermal characteristics in these days.
Further, as an alternative solution to this problem, there were presented theories and experimental equations for measuring the thermal resistance of the coated graphite. However, the theories and experimental equations are not compatible with actually obtained experimental data. This makes it difficult to apply the theories and experimental equations to actual material designs.
FIG. 1 is a perspective view showing a conventional thermal diffusivity measurement device 100. The thermal diffusivity measurement device 100 largely includes, as shown in FIG. 1, the first and second sample holder plates 12 and 14, sample holders 16, a measurement sample 10, a sample cover 18 and peripheral measurement units.
The sample holder 16 is a member for holding the measurement sample 10. The sample holder 16 is made of steel materials and is placed between the second sample holder plates 12. The first sample holder plates 12 are positioned on the second sample holder plates 14 on both sides of the sample holder 16, thus fixing the sample holder 16.
Further, the sample cover 18 is placed on the sample holder 16 such that it can be opened or closed when the measurement sample 10 is inserted into or withdrawn from the sample holder 16. If a flash beam 30 is generated from a laser generator 60 in a state where the measurement sample 10 is disposed as described above, the flash beam 30 heats the measurement sample 10. This thermal diffusivity measurement device 100 is constructed to maintain an insulation state, and hence heat 40 dissipated from the measurement sample 10 is incident on an infrared sensor 70.
An output signal of the heat 40 dissipated from the infrared sensor 70 as described above is input to operation means 80 and is used to measure a half time. This operation means 80 are embedded software for detecting the output signal to calculate a half time and calculating thermal diffusivity on the basis of the operation result.
FIG. 2 is a graph showing a process of measuring a temperature change in the infrared sensor 70 as time elapses in the prior art. As shown in FIG. 2, a temperature measured in the measurement sample 40 shows a minute change at an early stage and, after a lapse of a certain time period, it reaches the highest temperature Tmax of the measurement sample 40. In this case, regarding the state of the measurement sample 10, it can be said that a temperature rise by the incoming flash beam 30 and a temperature drop by the outgoing heat 40 are in an equilibrium state. The time required for the half of the temperature rise to reach the thermal equilibrium is called the half time t1/2.
Equation (1) may represent a temperature rise at the rear surface of the measurement sample 10 according to a heating time,
                                          Δ            ⁢                                                  ⁢            T                                Δ            ⁢                                                  ⁢                          T              max                                      =                  1          +                      2            ⁡                          [                                                ∑                                      n                    =                    0                                    ∞                                ⁢                                                                            (                                              -                        1                                            )                                        2                                    ⁢                                      exp                    ⁡                                          (                                                                        -                                                      n                            2                                                                          ⁢                        2                        ⁢                                                                                                  ⁢                                                  π                          2                                                ⁢                        α                        ⁢                                                                                                  ⁢                                                  tl                          s                                                      -                            2                                                                                              )                                                                                  ]                                                          (        1        )            
where α and ls denote the thermal diffusivity α and the thickness of the measurement sample 10, respectively. ΔT denotes a temperature rise according to the time at the rear surface of the measurement sample 10 and ΔTmax denotes the peak level of a temperature rise at the rear surface of the measurement sample 10. Further, t denotes the illumination time of the flash beam.
A half of the time at which temperature rise ΔT at the rear surface of the measurement sample 10, after radiating the flash beam 30, reaches the thermal equilibrium state ΔTmax is called the half time t1/2. Further, the thermal diffusivity can be found according to Equation (2).
                    α        =                              0.138785            s            2                                t                          1              /              2                                                          (        2        )            
However, as mentioned above, if precise data of the thickness ls of the graphite layer is not obtained, there was a problem in that error occurs in the value of the thermal diffusivity.